ci: enable multicore builds for unit test (#3753)

This commit passes in an additional parameter to the cmake build
invocation which will allow the build process to run in parallel. This
should speed up builds

codebuild/bin/s2n_codebuild.sh

@@ -95,7 +95,7 @@ run_unit_tests() {
             -D${CMAKE_PQ_OPTION} \
             -DS2N_BLOCK_NONPORTABLE_OPTIMIZATIONS=True \
             -DBUILD_SHARED_LIBS=on
-    cmake --build ./build
+    cmake --build ./build -- -j $(nproc)
     test_linked_libcrypto ./build/bin/s2nc
     make -C build test ARGS=-j$(nproc);
 }